Multiple sclerosis (MS) is an inflammatory disease characterized by multiple foci of inflammation and infiltration of macrophages and encephalitogenic T cells in the central nervous system. There are two types of macrophages found within the CNS of MS patients: resident macrophages or microglia, which are also present in normal CNS, and inflammatory macrophages the migrate into the CNS from the periphery. Microglia have a resting non-activated phenotype, a very limited ability to stimulate encephalitogenic T cells, and are believed to play immunoregulatory and neuroprotective roles. Peripheral macrophages appear in the CNS during inflammation and these cells have a highly activated phenotype, efficiently stimulate expansion of encephalitogenic T cells, and are thought to contribute to neuronal tissue destruction.